Light bulk density sodium perborate

ABSTRACT

A light, bulk density (about 0.3-0.6 g./cc.) sodium perborate having a particle size larger than about 60 mesh is produced by reacting together an aqueous sodium metaborate solution with hydrogen peroxide in the presence of externally prepared, prismatic, seed crystals of sodium perborate tetrahydrate.

O United States Patent 1151 3,661,5 1 0 Winkley 1451 May 9, 1972 541 LIGHT BULK DENSITY SODIUM 2,947,602 8/1960 Youngman et al ..23/60 PERBORATE 3,348,907 10/1967 Pellens et a] ..23/60 [72] Inventor: Donald Charles Winkley, Trenton, NJ. FOREIGN PATENTS OR APPLICATIONS AssignooI FMC Corporation, New York, NY 1,436,629 3 1966 France ..23/60 [2 Filed: Ma.- 13 l,l42,304 GICHI Britain ..23/60 PP 8061979 Primary E.\'aminerl-lerbert T. Caner Attorney-Eugene G. Seems, Frank lanno and Milton Zucker [52] U.S. Cl ..23/60 51 Int. Cl. .C01b 15/12 ABSTRACT [58] Field of Search... ..23/60 A light, bulk density (about 03416 gJcc') Sodium perborate having a particle size larger than about 60 mesh is produced [56] References cued by reacting together an aqueous sodium metaborate solution UNITED STATES PATENTS with hydrogen peroxide in the presence of externally prepared, prismatic, seed crystals of sodium perborate 2,223,903 12/1940 von Drathen et al. ..23/60 tetrahydram 2,257,461 9/1941 Gilbert et al ...23/6O 2,828, l 83 3/1958 Celtimier et al. ..23/60 7 Claims, 2 Drawing Figures PNENTED MAY 9 I 8, 661 .51 O

PR6 AT SODlUM PERBORATE (lsx) H6 S I'AR HAPED INTOR SODIUM PERBORATE DONALD.C. WINKLEY (I5X) y e Wife?- LIGHT BULK DENSITY SODIUM PERBORATE BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a novel, low-bulk density sodium perborate and to the method of preparing same.

2. Description of the Prior Art It is known that sodium perborate can be produced by the reaction of sodium metaborate and hydrogen peroxide, and that this compound can be utilized in detergent products. Many of these detergent formulations contain spray-dried ingredients having means particle sizes of from about 30 to about 40 mesh and bulk densities of about 0.3 g./cc.; these are produced by spraying a liquid solution or slurry through a heated zone and recovering discrete particles of detergent ingredients. When these spray-dried particles are dry mixed with sodium perborate to form the detergent formulations, conventional perborate compounds segregate from the formulation because normally they have a higher bulk density (0.70-0.85 g./cc.) and further because of their extremely small particle size, normally below 60 mesh. This situation is further aggravated by the high friability of certain of these perborate products. That is, the particles break down during processing and handling to yield even finer particles, which further increases the segregation problem.

In an attempt to correct the situation, various methods have been proposed for producing low-density sodium perborate. One such method is set forth in British Patent 1,015,835 published on Jan. 5, 1966, in which the density of the sodium perborate is decreased by increasing the relative mole ratio of hydrogen peroxide to sodium metaborate. This process is operated at substantially low temperatures on the order of l-l5 C. to produce porous globules of the sodium perborate.

While this sodium perborate product is considered an improvement over perborate of the prior art, it nevertheless suffers the drawback of having to be produced at substantiallylow temperatures, i.e., to C., and offers only marginal improvements in friability. As a result, a new process has been desired for producing a sodium perborate product which can be carried out at substantially room temperature, or above, one which yields a sodium perborate having the desired size and bulk density required of detergent formulations, and one which is not friable, i.e., the particles are not subjected to breakdown on handling or processing.

SUMMARY OF THE INVENTION It has now been found that a unique form of sodium perborate crystals can be produced having a bulk density of from about 0.3 to about 0.6 g./cc. and a size larger than 60 mesh, said crystals being in the form of prismatic crystals having a lengthzwidth ratio of from 3:1 to 10:1 and radially clustered in a group having a common center with their ends protruding beyond the common center in a star"-appcaring pattern.

This unique, sodium perborate, star-shaped product is produced by reacting together, at -45 C., hydrogen peroxide and sodium metaborate in a molar ratio of H O :BO of 1:1 in the presence of externally prepared, preformed seed crystals of sodium perborate tetrahydrate having a lengthzwidth ratio of at least 3:1, and seperating the resulting sodium perborate crystals from their mother liquor.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a photomicrograph (15X magnification) of a sample of commercially available sodium perborate produced by prior art methods as defined in Example 3, Run B.

FIG. 2 is a photomicrograph (15X magnification) of a sample of the star-shaped sodium perborate of the present invention, prepared by the process set forth in Example 3, Run A.

DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS In carrying out the present process, an aqueous sodium metaborate solution must first be prepared. The sodium metaborate solution can be made up from sodium hydroxide and either borax or boric acid. When the sodium metaborate is prepared in this manner, the final solution normally is purified by filtration or by other known means to remove impurities. The concentration of the sodium metaborate in the aqueous solution is not critical; however, high concentrations, and preferably metaborate solutions which are saturated at the equivalent of 50-60 C., are preferred, since they reduce the quantity of water which must be removed in a subsequent, vacuum crystallization step.

The second ingredient, hydrogen peroxide, can be used in any concentration desired with 30-70% H 0 being preferred, since this reduces the amount of water which has to be removed during a subsequent crystallization step. The hydrogen peroxide may have therein stabilizers of various types, such as magnesium, stannate, silicate or organic stabilizers, such as ethylene diamine tetraacetic acid. These stabilizers, which may be present at levels of 10 to 1,000 ppm, do not adversely affect the use of such stabilized hydrogen peroxide in the present process.

In carrying out the present process, a conventional, vacuum crystallizing apparatus is charged with a saturated solution of sodium perborate, and preferably a solution which also contains some sodium metaborate, e.g., 0.3 sodium metaborate, which acts to reduce the solubility of the sodium perborate in solution. The sodium perborate can be prepared by simply mixing sodium metaborate and hydrogen peroxide or by simply dissolving conventional sodium perborate in water up to the limit of its solubility. This solution is maintained at a temperature of from 25-45 C. and is used as the mother liquor from which the desired sodium perborate is crystallized.

To this solution in the vacuum crystallizer is then added both hydrogen peroxide and a saturated solution of sodium metaborate such that the molar ratio of I-I O :BO is 1:1. Simultaneously, along with the hydrogen peroxide and sodium metaborate solution, are added preformed, seed crystals of sodium perborate tetrahydrate having a prismatic form and a length:width ratio of at least 3:1. These seed crystals, produced as set forth hereinafter, may be added to the mother liquor either continuously or in batches. The rate at which these seed crystals are added to the solution is expressed as the weight percent seeding rate, this rate being based on the weight of the final product (sodium perborate) produced from the solution. For example, if pounds per hour of sodium perborate product is produced and 20 pounds per hour of seeds are added, the seeding rate is 20 percent. In general, the seeding rate is maintained between 5 and 30 percent, with a preferred seeding rate of 10 to 25 percent being employed.

The reactants which are present in the crystallizer, namely sodium metaborate and hydrogen peroxide, are present in substantially a 1:1 molar ratio of H O :BO except for a slight excess sodium metaborate which is maintained in the mother liquor to decrease the solubility of the product, sodium perborate, in the final reaction medium. The concentration of sodium metaborate in the crystallizer solution can be expressed as grams per liter of excess sodium metaborate; that is, that amount of sodium metaborate which is in excess of the stoichiometric amount which will react with any hydrogen peroxide present to form sodium perborate. While the crystallization process is found to be substantially independent of the amount of excess sodium metaborate present within the range of 0-50 g./l., in the preferred operation of this invention, 15-30 g./l. of excess sodium metaborate are usually maintained in the reaction liquor. In addition, if desired, 5-7 g./l. of borax can be added to the reaction medium in order to help stabilize the hydrogen peroxide present and also for pH control. Normally, where some borax, as indicated above, is added to the reaction liquor, the combination of excess metaborate and borax form a well-buffered system, and under these conditions, the pH will range from about 10.0-10.5 and generally will be about 10.3 i 0.1.

Since the crystallization and recovery of the sodium perborate product takes place in a continuous manner, some sodium perborate crystals remain suspended in the mother liquor. It is desirable to maintain the slurry density of the mixture in the crystallizer at from 5 to 40 for optimum results; the slurry density is defined as the weight (in grams) of sodium perborate crystals per 100 ml. of slurry. The precise, slurry density necessary to obtain particles within the desired mesh size will depend on the product production rate and the seeding rate. For example, at a seeding rate of 20 percent and a production rate of 100 g./hr. of the star"-like product, a slurry density of 5 was found to produce satisfactory product. However, at a percent seeding rate, a slurry density of was required to prepare the same good quality product.

The sodium perborate star"-shaped crystals are prepared at a rate of between 50300 g./hr. per liter of active crystallizer volume, with a rate of 100 g./hr. per liter being preferred; the active crystallizer volume is the volume of slurry in the crystallizer wherein the crystals are uniformly distributed throughout the mother liquor. This production rate is obtained by regulating the amounts of the reactants which are added continuously to the crystallizer.

The reaction to form the sodium perborate is carried out in the crystallizer at temperatures of between 25-45 C. Temperatures below 25 C. tend to form finer size particles than desired, while temperatures above 45 C. cause excessive decomposition. By operating at temperatures of 25 C. or slightly above, commercial operation of this process is simplified because refrigerated cooling water does not have to be used to condense the water evaporated in commercial vacuum crystallizers normally employed in crystallizing the sodium perborate. The use of such temperatures, i.e., 25-45 C., is considered a marked advantage compared with other previous processes which require reaction temperatures no higher than about 15 C., because it obviates the need to employ refrigerating means to cool the crystallizer solution and/or condenser used to condense evaporated water from the crystallizer.

In operation, the reaction liquor in the crystallizer is maintained at a fixed temperature-to precipitate the crystals of sodium perborate under either atmospheric pressures or reduced pressures. Reduced pressures are preferred in order to facilitate evaporation of water from the reaction liquor in the crystallizer, and to aid in controlling the temperature of the reaction liquor. The removal of some water is desired to maintain the volume of reaction liquor constant, thereby obviating the need to sewer excess liquor containing reactants.

In the above process, preformed seed crystals of sodium perborate tetrahydrate having a prismatic form and having a length:width ratio of at least 3:1 must be added to the crystallizer, along with the hydrogen peroxide and sodium metaborate. These seed crystals are produced, in either a batch or a continuous crystallization operation, by adding concentrated sodium metaborate (about 5 M) and hydrogen peroxide having a concentration of from 15-70 percent to a saturated sodium perborate mother liquor having a temperature of from 25-45 C.; the preferred temperature for making the seed crystals is about 35 C. The crystals precipitate from solution at a rate of 25-300 g./hr. per liter and are separated from their mother liquor by filtering or by other known separating techniques.

In a typical procedure for the continuous production of sodium perborate tetrahydrate seed crystals, stoichiometric amounts of hydrogen peroxide and sodium metaborate solution are added to an agitator-equipped tankor crystallizer containing a saturated solution of sodium perborate. As additional hydrogen peroxide and metaborate solution are added to the crystallizer, perborate crystals precipitate, and some of these crystals are removed continuously for use as seed. The rate for producing sodium perborate seeds is between 25--300 g./l. per hour, with 150 g./l. per hour being preferred. This rate, i.e., 150 g./l. per hour, refers to the grams of seed crystals that are crystallized from one liter of active crystallizer volume each hour. The slurry density of the crystallizer liquor can range from 5-40. Generally, a high slurry density is preferred in order to obtain high production rates. For example, in order to obtain a 100 g./l. per hour production rate, a slurry density of 10-20 is preferred, i.e., 10 to 20 grams of sodium perborate per 100 ml. of active crystallizer volume.

In the production of seed crystals by the above continuous technique, both the sodium metaborate solution and hydrogen peroxide are added continuously. The crystallization takes place constantly with the constant addition of reactants, and the production rate varies, depending on the amounts of ingredients added. These should be regulated to yield from 25-300 g., and preferably 150 g. of seed crystals per hour per liter of active crystallizer volume. In batch-type, seed crystal production, this addition time should range from 45 minutes to 2 k hours to permit the crystallization of abatch over this period of time.

The particle shape of these seed crystals is extremely important to the overall process. Generally, the seed crystals are long, needle-like crystals, i.e., prismatic crystals, having a lengthzwidth ratio of between 3: 1-10: 1. These crystals can be individual, prismatic crystals or may be grouped together in clusters having a common center with protruding ends to form small, star"-shape configurations. The number of rectangular, needle-like crystals which make up these star"-shaped clusters can range from between 2 and 20.

The following examples are given to illustrate the present invention and are not deemed to be limiting thereof.

EXAMPLE 1 CONTINUOUS PREPARATION OF SODIUM PERBORATE SEED CRYSTALS A 3 /2 liter, vacuum crystallizer was charged with water. The temperature of the water in the crystallizer was 35 C., and it was maintained at this temperature throughout the entire run. An aqueous solution containing 35 percent hydrogen peroxide and an aqueous solution containing 5 M sodium metaborate were both added continuously to the crystallizer, and water was evaporated continuously from the crystallizer until a saturated solution of sodium perborate was produced by the reaction of the hydrogen peroxide and sodium metaborate. To the resulting, saturated solution in the crystallizer, which had a volume of about 3 liters, was added 5 g./l. of borax and 20 g./l. of sodium metaborate. Thereafter, additional hydrogen peroxide solution and sodium metaborate solution were added to the crystallizer in a 1:1 molar ratio of H O zBoand at a rate sufficient to produce g. of sodium perborate per hour per liter of mixture in the crystallizer.

Sodium perborate crystals were removed every 15 minutes during the run at a rate corresponding to 100 g./l. per hour. The product was isolated by passing the mother liquor slurry through a vacuum filtration stage where the sodium perborate crystals were separated and then returning the filtered mother liquor back to the crystallizer. Sufficient water was evaporated in the vacuum crystallizer to maintain the level of mother liquor constant so that no mother liquor was discarded during the run.

The resulting sodium perborate tetrahydrate seed crystals were found to hem the form of prismatic crystals having a length:width ratio of above 3:1. Many of these small crystals were clustered around a common center having protruding ends beyond the common center and giving a star"-like appearance. The bulk density of these sodium perborate crystals was 0.36 g./cc. and the range of particle size was 34-91 mesh. These sodium perborate tetrahydrate crystals were used as seed crystals in subsequent example 3, Run A.

EXAMPLE 2 BATCH PREPARATION OF SODIUM PERBORATE SEED CRYSTALS A 3 b liter, vacuum crystallizer similar to that used in Example l was charged with a saturated, aqueous solution of sodium perborate maintained at 35 C. The saturated solution had a volume of about 3 liters and contained 5 g./l. of borax and 20 g./l. of excess sodium metaborate; that is, excess sodium metaborate over the stoichiometric amount necessary to form sodium perborate with any H O in solution. An aqueous solution containing 35 percent hydrogen peroxide and a 5 M sodium metaborate solution were then both continuously added to the vacuum crystallizer at a rate sufiieient to produce 100 g. of sodium perborate tetrahydrate crystals per hour per liter of slurry in the crystallizer. The aqueous hydrogen peroxide and aqueous sodium metaborate solution were added in a ratio sufficient to have a 1:1 molar ratio of 11 0 30 After continuous addition for about 30 minutes, small crystals of sodium perborate tetrahydrate commenced to form. The addition of the solutions was continued for an additional hour, making a total addition time of 1 hour and 30 minutes. When the slurry density reached 15 (the slurry density being defined as the weight in grams of sodium perborate tetrahydrate crystals per 100 ml. of slurry in the crystallizer), the contents of the crystallizer were filtered, and the resulting crystals separated from their mother liquor.

This procedure was repeated to obtain several 1.5-hour batch crystallizations of sodium perborate tetrahydrate crystals. The resulting crystals had a bulk density of 0.45 g./cc. The crystals were prismatic, needle-like crystals having a lengthzwidth ratio of above 3:]. While a few of the crystals were single, prismatic crystals, the major form was clusters of from 2 to of these needle-like crystals having common centers and the ends protruding beyond the common center to yield a star"1ike structure. These crystals were used as seed crystals in example 4.

EXAMPLE 3, RUN A PREPARATION OF STAR- SHAPED SODIUM PERBORATE TETRAI-IYDRATE CRYSTALS USING CONTINUOUSLY PREPARED SEEDS A 3 A liter, vacuum crystallizer was charged with a saturated, aqueous solution of sodium perborate having a temperature of 35 C. The saturated solution which had a volume of about 3 B liters also contained 0.8 g./l. of borax and 16 g./l. of excess sodium metaborate (sodium metaborate in amounts beyond the stoichiometric amounts required to produce sodium perborate with any hydrogen peroxide which is present in solution). An aqueous solution containing 35 percent hydrogen peroxide and an aqueous solution of 5.4 M sodium metaborate were both added continuously to the crystallizer at a rate sufficient to produce 100 g. of sodium perborate tetrahydrate per hour per liter of slurry in the crystallizer. Sodium perborate tetrahydrate seed crystals prepared in example were also added continuously with the sodium metaborate and the hydrogen peroxide solutions so as to achieve a seeding rate of 16.7 percent. This required adding 20 g. of seeds per hour per liter of slurry. The seeding rate is the weight of sodium perborate tetrahydrate seeds which are added per hour divided by the weight of the final product which is produced per hour from a crystallizer. Accordingly, for every 100 g./hr. of sodium perborate tetrahydrate product recovered, 16.7 g./hr. of seeds were added to the crystallizer.

The sodium perborate tetrahydrate product was removed from the crystallizer at a rate of about 120 g./hr. per liter of slurry in the crystallizer. This was done by constantly passing the mother liquor and crystal slurry through a vacuum filter, separating the crystals from the mother liquor and returning the mother liquor back to the crystallizer. The slurry density (weight in grams of crystals per 100 ml. of slurry) of the liquor in the crystallizer was 10, and sufiicient water was removed under vacuum to maintain the volume of the mother liquor constant throughout the run.

The crystallizer was operated for several hours until a steady state had been reached. The resulting sodium perborate tetrahydrate product which was recovered was found to have a bulk density of about 0.47 g./cc., a mean particle size of 29 mesh and to be in the form of prismatic crystals with a lengthzwidth ratio of 3:1 to 6:1. The prismatic crystals were clustered into groups having common centers with protruding ends beyond the common centers to give a star"-like appearance. These crystals are shown in FIG. 2 of the drawings which are photomicrographs of the crystals under a 15x magnification.

Run B By way of comparison, FIG. 1 of the drawings illustrates photomicrographs of conventional sodium perborate crystals prepared by mixing together hydrogen peroxide and sodium metaborate at substantially room temperature, except that no sodium perborate tetrahydrate seeds of the type described in the present invention were added to the crystallizing solution. Rather, the product was separated from its mother liquor, and the motherliquor simply recycled back to the crystallizer. As will be readily apparent, these are in the shape of small, irregular, squat, columnar crystals having a bulk density of from about 0.7 to about 0.8 g./cc.

EXAMPLE 4 PRODUCTION OF STAR-SI-IAPED SODIUM PERBORATE TETRAHYDRATE CRYSTALS USING BATCH-WISE PREPARED SEEDS The procedure of example 3 was repeated, except that the seed crystals which were added were those prepared as set forth in example 2. The seed crystals were added in the I amount of 20 g./hr. to yield a seeding rate of 16.7 percent. In

this case, however, the slurry density in the crystallizer was 15 instead of 10 as in example 3. The solution of mother liquor in the crystallizer analyzed 35 g./l. of sodium perborate, 16 g./l. of excess sodium metaborate (the amount in excess of the stoichiometric amount required to react with any hydrogen peroxide to produce sodium perborate) and 0.8 g./l. of borax. A state of steady operation was reached in the crystallizer after about 2 to 3 hours, and the sodium perborate tetrahydrate product which was recovered had a bulk density of 0.45 g./cc., a mean particle size of 21 mesh and a friability of 8 percent. The friability is determined as follows: 20 g. of mesh'product are placed in a 1 inch I.D. vertical tube, and a gas is passed upwardly through the tube at a velocity of 7.07 1. per minute for 2 minutes. During this period the product is suspended by the gas and subject to multiple collisions while so suspended. The proportion of -l00 mesh particles that result from this treatment is then determined.

The final sodium perborate tetrahydrate product was made up of prismatic crystals having a lengthzwidth ratio of 3:1 to 7:1; clusters of from 4 to 20 of these prismatic crystals were grouped together about a common center with protruding ends beyond the common center to yield a star"-like cluster.

EXAMPLE 5 To produce a star"-shaped sodium perborate monohydrate product the following procedure was utilized. A 100 gram sample of the star"-shaped sodium perborate tetrahydrate product produced from example 3, Run A was suspended on a screen in a container and forced, dry, heated air was passed through the container and through the sample on the screen for about 30 minutes until the sample was completely dehydrated to the monohydrate. The temperature of the sample during the conversion was about 50 C. The resulting, star-shaped sodium perborate monohydrate had the same gross morphology as the precursor crystals and its bulk density was 0.39 g./cc.

EXAMPLE 6 The star"-shaped sodium perborate tetrahydrate can also be converted into other sodium perborate products, such as effervescent sodium perborate, which has the property of giving off gaseous oxygen on contacting water. This conversion was carried out as follows: a 100 gram sample of "starshaped sodium perborate tetrahydrate product produced as set forth in example 3, Run A was placed in a vertical tube and dry, hot air was passed upward through the tube at a superficial, linear velocity of about 2 feet per second. The sample was suspended by the air stream and while suspended was heated to 150 C. by means of a heating jacket surrounding the vertical tube and was maintained at this temperature for minutes, A star-shaped, effervescent sodium perborate product was obtained that had the same gross morphology as the precursor crystals, and its bulk density was 0.31 g./cc. The product was found to have an active oxygen content of 8.6 percent and a gaseous oxygen content of 7.4 percent (weight percent of 0, gas liberated on contact with water) and to have the efiervescent properties described in U.S. Pat. No. 3,421,842 issued to Darbee et al. on Jan. 14, 1969.

From the above examples 5 and 6 it is clear that a starshaped sodium perborate product can be produced in the form of tetrahydrate or monohydrate crystals or as a sodium perborate product containing active oxygen and/or gaseous oxygen. It is intended that the term sodium perborate, as used in the specification and claims, includes all hydrates of sodium perborate containing from 1 to 4 waters of crystallization, and mixtures thereof, as well as sodium perborate that contains active oxygen and/or gaseous oxygen (as defined in US, Pat. No. 3,421,842).

Pursuant to the requirements of the Patent Statutes, the principle of this invention has been explained and exemplified in a manner so that it can be readily practiced by those skilled in the art, such exemplification including what is considered to represent the best embodiment of the invention. However, it should be clearly understood that, within the scope of the appended claims, the invention may be practiced by those skilled in the art, and having the benefit of this disclosure otherwise than as specifically described and exemplified herein.

WHAT IS CLAIMED IS:

I. As a novel composition, sodium perborate prismatic crystals having a lengthzwidth ratio of 3:1 to 10:1, said crystals grouped together in clusters have a common center with ends protruding from said common center to form star"-shaped clusters, said clusters having a size greater than 60 mesh and a bulk density of from about 0.3 to about 0.6 g./cc, prepared by the process of claim 2.

2. Process of producing sodium perborate in the form of prismatic crystals having a lengthzwidth ratio of 3:1 to 10:], said crystals being radially clustered about a common center in a star-shaped cluster having a size larger than 60 mesh and a bulk density of 0.3-0.6 g./cc. which comprises reacting together at 25-45 C., in an aqueous medium, hydrogen peroxide and sodium metaborate in a mole ratio of H,O,:BO, of 1:1 in the presence of externally prepared, preformed seed crystals of sodium perborate tetrahydrate having a smaller size than the desired product and a lengthzwidth ratio of at least 3:] and separating the resultant sodium perborate crystals from their mother liquor.

3.. Process of claim 2 wherein the hydrogen peroxide is added as an aqueous solution having a concentration of 30-70 percent hydrogen peroxide and the sodium metaborate is added as an aqueous solution containing about 5 M sodium metaborate.

4. Process of claim 2 wherein the process is carried out in a vacuum crystallizer and water is constantly evaporated from the reaction solution to maintain the volume of the solution constant.

5. Process of claim 2 wherein the seeding rate is maintained from 5 to 30 percent and the sodium perborate star-shaped crystals are prepared at a rate of between 50-300 g./hr. per liter of active crystallizer volume.

6. Process of claim 2 wherein the reaction takes place in the presence of 15-30 g./l. of excess sodium metaborate and 5-7 g./l. of borax.

7. Process of claim 2 wherein the slurry density of the reaction mixture is from 5-40.

l i i 

2. Process of producing sodium perborate in the form of prismatic crystals having a length:width ratio of 3:1 to 10:1, said crystals being radially clustered about a common center in a ''''star''''-shaped cluster having a size larger than 60 mesh and a bulk density of 0.3- 0.6 g./cc. which comprises reacting together at 25* - 45* C., in an aqueous medium, hydrogen peroxide and sodium metaborate in a mole ratio of H2O2:BO2 of 1:1 in the presence of externally prepared, preformed seed crystals of sodium perborate tetrahydrate having a smaller size than the desired product and a length:width ratio of at least 3:1 and separating the resultant sodium perborate crystals from their mother liquor.
 3. Process of claim 2 wherein the hydrogen peroxide is added as an aqueous solution having a concentration of 30- 70 percent hydrogen peroxide and the sodium metaborate is added as an aqueous solution containing about 5 M sodium metaborate.
 4. Process of claim 2 wherein the process is carried out in a vacuum crystallizer and water is constantly evaporated from the reaction solution to maintain the volume of the solution constant.
 5. Process of claim 2 wherein the seeding rate is maintained from 5 to 30 percent and the sodium perborate ''''star''''-shaped crystals are prepared at a rate of between 50- 300 g./hr. per liter of active crystallizer volume.
 6. Process of claim 2 wherein the reaction takes place in the presence of 15- 30 g./l. of excess sodium metaborate and 5- 7 g./l. of borax.
 7. Process of claim 2 wherein the slurry density of the reaction mixture is from 5-
 40. 